Non-volatile memory (“NVM”) refers to semiconductor memory which is able to continually store information even when the supply of electricity is removed from the device containing the NVM cell. NVM includes Mask Read-Only Memory (Mask ROM), Programmable Read-Only Memory (PROM), Erasable Programmable Read-Only Memory (EPROM), Electrically Erasable Programmable Read-Only Memory (EEPROM), and Flash Memory. Non-volatile memory is extensively used in the semiconductor industry and is a class of memory developed to prevent loss of programmed data. Typically, non-volatile memory can be programmed, read and/or erased based on the device's end-use requirements, and the programmed data can be stored for a long period of time.
As the information technology market has grown vastly in the past twenty years or so, portable computers and the electronic communications industry have become the main driving force for semiconductor VLSI (very large scale integration) and ULSI (ultra large scale integration) design. As a result, low power consumption, high density and re-programmable non-volatile memory are in great demand. These types of programmable and erasable memories have become essential devices in the semiconductor industry.
A rising demand for memory capacity has translated into higher requirements for integration level and memory density. Dual bit cells which can store two bits of information in each memory cell are known in the art but are not yet prevalent in use. Some dual bit cells have multiple threshold voltage levels, where every two threshold voltage levels together store a different bit. These types of dual bit cells involve operational complexities which discourage their widespread use. Other dual bit cells have two separate storage sites and store one bit in each site on either side of the cell. One kind of dual bit cell of the latter variety is known as Nitride Read Only Memory (NROM).
Nitride read only memory is a type of charge-trapping semiconductor device for data storage. In general, an NROM cell is composed of a MOSFET (metal-oxide-silicon field effect transistor) having an ONO (oxide-nitride-oxide) gate dielectric layer disposed between the gate and the source/drain semiconductor material. The nitride layer in the ONO gate dielectric layer is able to trap electrons in a localized manner when programmed. Charge localization refers to the nitride material's ability to store the charge without much lateral movement of the charge throughout the nitride layer. This is in contrast to conventional floating gate technology wherein the floating gate is conductive and the charge is spread laterally throughout the entire floating gate. Programming (i.e., charge injection) of the charge-trapping layer in NROM devices can be carried out via channel hot electron (“CHE”) injection. Erasing (i.e., charge removal) in NROM devices can be carried out via band-to-band hot hole tunneling. The stored charge can be repeatedly programmed, read, erased and/or reprogrammed via known voltage application techniques, and reading can be carried out in a forward or reverse direction. Localized charge-trapping technology allows two separate bits per cell, thus doubling memory density.
Although localized charge-trapping read only memory, such as NROM, has the advantage of two-bit storage in each cell, the constant industry demand to reduce overall memory cell dimensions has adverse implications for this technology. The trend in integrated circuit manufacture to produce memory cells with reduced feature sizes can result in unwanted phenomena. This is particularly true for MOSFETs as one of the dimensions thus reduced is channel length (i.e., the distance between the source and drain regions). As MOSFET channel length is reduced, charges in depletion regions near the source and/or drain may link with charges in the channel region, thereby skewing the threshold voltage, increasing the occurrence of unwanted “punch-through” and altering other device characteristics of the MOSFET. These effects are collectively known as “short-channel effects.” A number of prior art devices have been proposed to address the short-channel effects. Some have proposed reducing the dimensions of the source and drain depletion regions. Such a reduction, however, has an unintended adverse effect of increasing bit-line (source/drain) resistance, which in turn can affect the voltage-current characteristics of the device and/or increase the heat produced by the device.
Another problem encountered in localized charge-trapping dual bit cells, such as NROM cells, is the so-called “second-bit effect”, which can become more pronounced in short channel devices. The second-bit effect refers to the adverse effects one stored bit (trapped charge) has on the manipulation (e.g., programming and/or reading) of the other bit. For example, in some cases, when programming a second bit by channel hot electron injection there is a possibility that some electrons will be unintentionally injected into the charge-trapping layer at the first bit location, especially where the channel length is short and the first and second bits are closer to each other. As a result of this type of unintentional electron injection, the already programmed first bit can be “overwritten” (i.e., over-programmed), which in turn affects the width of the depletion layer under the expanded bit. As channel length decreases the possibility of overwriting increases. In addition, the overwriting effect described above also can lead to the second-bit effect characterized by alteration of the threshold voltage of the other bit during its reading operation.
Another problem encountered in memory devices, such as those which use channel hot electron programming and/or band-to-band hot hole erasing methods, is breakdown between the source/drain regions and gate, which results from the high voltage differential between the source/drain regions and the gate. Buried-diffusion oxide materials deposited above the source/drain regions have been used to decrease the possibility of breakdown between the source/drain regions and the gate. However, as memory cells become smaller and smaller, conventional buried-diffusion oxides become less capable of preventing or diminishing breakdown because their dielectric properties are insufficient at decreased dimensions.
Despite these problems encountered with the scale down of charge-trapping memory cells, interest in their production, architecture and use continues to grow. It is expected that NROM technology will eventually compete with conventional floating gate technology for many NVM applications. Accordingly, it is desirable to provide a charge-trapping NVM cell structure capable of decreased dimensioning and to provide a method for manufacture of such an NVM cell in which short-channel effects, the second-bit effect and breakdown between the source/drain and gate are minimized.